1. Field of the Invention
The present invention relates to gyroscopes, and in particular to improved resonator microgyroscopes and their manufacture. More particularly, this invention relates to producing and tuning microgyroscopes, and particularly, isolated resonator gyroscopes.
2. Description of the Related Art
Gyroscopes are used to determine direction based upon the sensed inertial reaction of a moving mass. In various forms they are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to autonomously determine the orientation of a free object.
Older conventional gyroscopes were very heavy mechanisms, employing relatively large spinning masses by current standards. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes as well as vibratory gyroscopes.
Spacecraft generally depend on inertial rate sensing equipment to supplement attitude control. Currently this is often performed with expensive conventional spinning mass gyros (e.g., a Kearfott inertial reference unit) or conventionally-machined hemispherical resonator gyroscopes (e.g. a Litton hemispheric resonator gyroscope inertial reference unit). However, both of these are very expensive, large and heavy.
In addition, although some prior symmetric vibratory gyroscopes have been produced, their vibratory momentum is transferred directly to their baseplates or packages. This transfer or coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang et al. which describes a symmetric cloverleaf vibratory gyroscope design and is hereby incorporated by reference herein. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate, however these gyroscopes lack the vibrational symmetry desirable for tuned operation.
In addition, shell mode gyroscopes, such as the hemispherical resonator gyroscope and the vibrating ring gyroscope, are known to have some desirable isolation and vibrational symmetry attributes, however, these designs are not suitable for or have significant limitations with thin planar silicon microfabrication. The hemispherical resonator employs the extensive cylindrical sides of the hemisphere for sensitive electrostatic sensors and effective actuators, however its high aspect ratio, 3D curved geometry is unsuitable for inexpensive thin planar silicon microfabrication. The thin ring gyroscope while suitable for planar silicon microfabrication lacks electrostatic sensor and actuators that take advantage of the extensive planar area of the device.
Vibration isolation using a low-frequency seismic support is also known (e.g., U.S. Pat. No. 6,009,751, which is incorporated by reference herein), however such increased isolation comes at the expense of proportionately heavier seismic mass and/or lower support frequency. Both effects are undesirable for compact tactical inertial measurement unit (IMU) applications.
Furthermore, the scale of previous silicon microgyroscopes (e.g., U.S. Pat. No. 5,894,090) has not been optimized for navigation grade performance resulting in higher noise and drift than desired. This problem stems from a use of thin epitaxially grown silicon flexures to define critical vibration frequencies that are limited to 0.1% thickness accuracy. Consequently device sizes are limited to a few millimeters. Such designs exhibit high drift due to vibrational asymmetry or unbalance and high rate noise due to lower mass which increases thermal mechanical noise and lower capacitance sensor area which increases rate errors sensor electronics noise.
Scaling up of non-isolated silicon microgyros is also problematic because external energy losses will increase with no improvement in resonator Q and no reduction in case-sensitive drift. An isolated cm-scale resonator with many orders of magnitude improvement in 3D manufacturing precision is required for navigation grade performance.
Conventionally machined navigation grade resonators such as in hemispherical or shell gyros have the optimum scale, e.g. 30 mm and 3D manufacturing precision and hence desirable drift and noise performance, however such gyros are expensive and slow to manufacture. Conventional laser trimming of mechanical resonators can further improve manufacturing precision to some degree, however this process is not suitable for microgyros with narrow mechanical gaps and has limited resolution, necessitating larger electrostatic bias adjustments in the final tuning process.
There is a need in the art for small gyroscopes with greatly improved performance for navigation and spacecraft payload pointing. There is also a need for such gyros to be cheaper and more easily manufactured with greater 3D mechanical precision. There is still further a need for such gyros to have desirable isolation and vibrational symmetry attributes while being compatible with planar silicon manufacturing. Finally, there is a need for such gyros to have robust designs, easily tunable and capable of selective control of differential rocking modes and balance control. The present invention satisfies all these needs.